Gravure coating processes have been established and are well known within the industry, as they provide a method to coat a flexible substrate in a uniform manner. Complete coating of the substrate is common among many of these applications, particularly in printing and priming industries. However, complete coating is not advantageous in some other processes, particularly when the material is subjected to further stretching.
Biaxial orientation, which involves sequential or simultaneous machine and transverse direction stretching is common in the production of some polymer substrates (polyethylene terephthalate, polypropylene, polylactic acid, etc.) as the orientation imparts desired properties that are useful for packaging films and many industrial films. Transverse stretching is commonly done by a tenter process within the industry, wherein the edges of the film are captured by clips on a moving chain that will then stretch the film to a desired width before releasing it. In some cases, it is desirable to coat one or both sides of the substrate to provide useful properties such as gas barrier, printability, heat sealability, etc. It can be also desirable in some cases to apply the coating “in-line” in a biaxial orientation process, typically between the machine direction orientation process (MDO) and the transverse orientation process (TDO) in a sequential orientation line. (In a simultaneous orientation line, such a coating process can be placed after the extrusion and casting section but prior to the tentering oven.) Typically, a coating station is placed between the MDO and TDO and the coating may be applied by any means well-known in the art, including but not limited to gravure coating, rod coating, slot die coating, etc. Preferred is gravure roll coating. In order to avoid build-up of coating and a detrimental effect to machine performance, coating close to the thick edges of the MD-oriented sheet is avoided.
There are many configurations for gravure coating a substrate, but most can be described within two application categories: standard configurations where a backing roll places the substrate in contact with the gravure roll or offset roll; or kiss-coat configurations (see FIGS. 1 and 2) where the substrate is wrapped around the gravure roll or offset roll without the use of a backing roll. U.S. Pat. No. 3,844,813 to Leonard et al. describes these gravure coating processes onto a textile substrate. The '813 patent, however, does not disclose any edge masking systems that may be employed in the process.
Edge masking can be employed by current commercial systems provided by machinery manufacturers such as Davis Standard, LLC of Pawcatuck, Conn. Commercial designs for a conventional gravure method include manufacturing the backing roll in a way to provide a two-stepped roll diameter so that the edges of the substrate are not pressed into the gravure roll, and therefore not coated, due to a smaller roll diameter on the edges. One obvious drawback in this design is that the uncoated region can only be adjusted by changing the backing roll, necessitating significant downtime for every width change in the substrate. Additionally, in certain applications, using a backing roll can be very difficult as slight misalignments or speed differences can cause wrinkles or other defects into the finished product. Defects can also occur when running in reverse gravure (when the gravure roll runs counter to the film direction), and slight coating fluid property differences fail to provide the necessary lubricity. These defects can be particularly detrimental in further stretching processes and can cause complete failure within the process (i.e. film breaks).
Commercial designs for kiss-coat edge masking include use of a thin (40 mil or 1000 μm) TEFLON® (PTFE or polytetrafluoroethylene) sheet at custom widths (typically 2-4 inches or 5-10 cm) that is wound on spools before and after the gravure roll. The wound material can be manually indexed to account for wear. While this system does mask the edges it fails to provide long-term abrasion resistance with indexing potentially necessary every 30-60 minutes depending on the substrate. The indexing is also limited by the total amount of material that can be wound, necessitating a more involved change every 1-2 days with substantial downtimes incurred. Additionally, this design is in a fixed transverse location so that any product width changes require a reconfiguration of the hardware with additional downtime incurred. This lack of transverse or side-to-side adjustment flexibility while in production mode is very limiting and unproductive.
Wear to the masking material with the commercial design can be exacerbated when using gravure coating substrates with thick edges, particularly with in-line coating methods used in biaxial orientation film manufacturing. As described earlier, in many polymer orientation processes the substrate is captured within a clip system for transverse or simultaneous machine and transverse stretching. To withstand this clip system, finished thin films (less than 5 mil or 125 μm) require that the edges of the film be much thicker than the middle of the film substrate. This thick edge portion is typically about 40-50 mm wide and is the portion of the film that goes inside the tenter chain clip. The tenter chain clip then closes upon this edge portion and is the method by which the film is conveyed into the tentering oven. This edge thickness can cause excessive wear on the commercial edge masking, necessitating very frequent indexing of the masking material and eventual production losses as downtime is needed to replenish the masking material and/or clean-up of coating build-up.
Additionally, with transverse stretching of polymers, the transition between uncoated substrate and coated substrate can be exceptionally difficult to manage for stability of the process. If the desired coating thickness is high (high stretch ratio, low solids, etc), the temperature difference between the coated and uncoated polymer within the tenter oven, which performs a dual function of both drying the wet coating as well as heating the substrate to enable orientation, can lead to overstretching failure of the uncoated portion within the process. Precise management of this transition from coated to uncoated regions is desired for a stable process in this instance. With polymer stretching, slight differences in width between and during runs are common after machine direction stretching, up to 3% of the width of the MD-oriented film. This makes any fixed masking setup difficult to manage for a stable process. Thus, the masking system should have the ability to be adjusted side-to-side (or transversely) to effectively mask the edges and prevent coating of the edges if such width variations occur.
The current invention addresses the above deficiencies regarding wear and coating width management.